16 Mr. E. C. Nourse on the Colours of Leaves and Petals. 
IV.—On the Colours of Leaves and Petals. By Wit1iam 
E. C. Nourss, M.R.C.S, 
Tue colours of leaves and petals depend on several conditions ; 
some mechanical or structural, and some chemical. The latter 
have been made the subject of many investigations. The former, 
though requiring little more than common observation, have 
been passed over, or but slightly noticed. It is to a clearer 
knowledge of these that the present paper, so far as it goes, is 
intended to contribute. 
The structural or mechanical circumstances which influence 
the colours are, Ist, the situation of the coloured cells ; 2nd, their 
size, form and number ; 3rd, their mixture with each other; and 
4th, their degree of visibility. 
1. The situation of the coloured cells is different in leaves and 
petals, though their general structure is anatomically similar. If 
a leaf be torn, the green colour appears to be in the central sub- 
stance ; while in a petal the centre is nearly white, and the co- 
lour peels off with the cuticle. This difference merits a degree 
of attention which has not hitherto been paid to it. 
The structural parts of a leaf or petal, are, the substance, con- 
sisting of cellular tissue and venous ramifications ; the cuticle, or 
epidermis ; and a layer of cells immediately beneath the cuticle, 
to which we may give the provisional name of Refe. 
This latter structure is seldom mechanically distinct, but is 
either continuous with the substance, as in leaves, or adherent 
to the cuticle, as in petals ; though it can sometimes be shown in 
a separate form in petals of a large size. Its characters, however, 
effectually distinguish it from the other structures. It is the 
densest parenchyma of the organ, consisting of an immense 
number of nearly circular cells without any interstices. But the 
circumstance which makes it most conspicuous is, that it is the 
seat of colouring matters which are scarcely found in the other 
structures. The rete thus appears entitled to be considered as a 
distinct tissue, and may be found to perform important fune- 
tions, especially in the petals, in which its development is most 
distinct. 
The colours of the rete possess an almost endless variety, and, 
in fact, it is in this structure that the most highly coloured cells 
are always found. Of petals, it contains the entire colouring ; the 
yellow, red, blue, brown, black, and all the intermediate tints are 
wholly produced in its cells, and can be completely removed by 
simply stripping it off with the cuticle. This can be easily done 
with any common flower. In leaves the rete is the seat of all the 
modifications of the green colour which those organs present, 
excepting variegation, cuticular changes, and what may be called 
